1. Field of the Invention
The present invention relates to orthopaedic devices, and, more particularly, to orthopaedic implants.
2. Description of the Related Art
Most orthopaedic implants are formed from a metallic material suitable for a given implant, such as a hip implant, knee implant, glenoid implant, etc. In the case of articulating joints, the implant may include a non-metallic load bearing surface, such as an ultra high molecular weight polyethylene (UHMWPE). The UHMWPE is attached to the metallic body of the implant, and provides the implant with good wear characteristics and low friction.
It is also known to provide an implant with a porous bony ingrowth surface. For example, a hip implant may include a porous surface on the stem which is intended to allow bony ingrowth of the proximal end of the femur bone. Such a porous surface may be in the form of a metal porous surface which is bonded, such as by heat sintering, to the stem of the implant. Examples of porous surfaces of this type include a woven mesh, a fiber mesh and particles. Knee implants are also known that include porous ingrowth surfaces that can bear load from surrounding anatomic structures.
Porous surfaces of the type described above which are used with implants are not typically part of a single structural member with two opposed, external porous surfaces. For example, in a knee implant, the distal surface of the implant can sit on the porous material that is slightly above the substrate material, but the porous material only typically has one external surface for tissue ingrowth. For hip implants, the porous ingrowth surface is usually provided as a coating on a structural component of the implant, such as the stem.
In some orthopaedic applications, such as spinal cages, it is beneficial to have a porous member that extends between two external, load bearing surfaces of the implant. In such arrangements, a cavity is typically formed between the two external surfaces of the implant and filled with a porous ingrowth material, which is typically a natural substance such as cancellous bone tissue. Such an implant is described in U.S. Patent Application No. 2002/0091447 to Shimp et al. One problem with the implant described by Shimp et al. is that harvesting sufficient cancellous bone tissue to fill the cavity is expensive, and host rejection issues can be a concern. Other similar implants that contemplate utilizing natural or synthetic materials are described in U.S. Patent Application Publication No. 2004/0210316 to King et al., and U.S. Pat. No. 6,423,095 to Van Hoeck et al. In each of these described implants, the porous material held in the cavity is fairly isolated from bearing load from surrounding anatomic structures after implantation, with external surfaces that are either flush or below the most protruding external surface of the main implant body. This is intentional, as the materials placed in the cavity tend to have significantly lower strength than the implant body. However, isolating the porous ingrowth material from bearing loads from surrounding anatomic structures also decreases the amount of surface area the porous ingrowth material has in contact with the anatomic structures, which can slow down integration of the implant. In addition, the porous materials placed in the cavity are typically resorbable by the body and will not last throughout the life of the implant.
What is needed in the art is an orthopaedic implant that can overcome some of the disadvantages of known devices.